FIG. 7 is a cross-sectional view showing the structure of a prior art visible laser diode. In FIG. 7, an n type GaAs buffer layer 12 is disposed on an n type GaAs substrate 11. An n type AlGaInP cladding layer 13 is disposed on the n type GaAs buffer layer 12. An undoped GaInP active layer 14 is disposed on the n type AlGaInP cladding layer 13. A p type AlGaInP waveguide layer 15 is disposed on the undoped GaInP active layer 14. A p type GaInP etch stopping layer 16 is disposed on the p type AlGaInP waveguide layer 15. A p type AlGaInP cladding layer 17 is disposed on the p type GaInP etch stopping layer 16. A p type GaInP layer 18 is disposed on the p type AlGaInP cladding layer 17. A p type GaAs contact layer 20 is disposed on the p type GaInP layer 18. A forward mesa stripe is formed of the p type AlGaInP cladding layer 17, p type GaInP layer 18 and a part of p type GaAs contact layer 20. An n type GaAs current blocking layer 19 is disposed on the p type GaInP etch stopping layer 16 to bury the forward mesa stripe.
A description is given of the production process.
First of all, an n type GaAs buffer layer 12, an n type AlGaInP cladding layer 13, an undoped GaInP active layer 14, a p type AlGaInP waveguide layer 15, a p type GaInP etch stopping layer 16, a p type AlGaInP cladding layer 17, a p type GaInP layer 18 and a p type GaAs cap layer 20a are successively grown on an n type GaAs substrate 11 by MOCVD (first crystal growth). Subsequently, a stripe pattern is formed using ordinary photolithography techniques and the p type GaAs cap layer 20a, p type GaInP layer 18 and p type AlGaInP cladding layer 17 are etched using the stripe pattern as a mask to form a mesa. Thereafter, an n type GaAs current blocking layer 19 is grown by MOCVD in the groove formed by the etching (second crystal growth), and a p type GaAs contact layer 20 as a top layer is grown by MOCVD (third crystal growth). Here, the p type GaAs cap layer 20a is fused into one with the p type GaAs contact layer 20 during the third growth, as shown by the dotted line in the p type GaAs contact layer 20 in FIG. 7.
In the laser diode as shown in FIG. 7, it is thought important to make the substrate and the epitaxial layer lattice-match with each other in order to suppress the crystalline defects and the deterioration of surface morphology, so that the lattice constant of the GaAs substrate coincide with the lattice constants of the GaInP and AlGaInP system layers as much as possible. In order to produce an improved laser diode, it is thought ideal to make the ratio between the lattice constant of the GaAs substrate and that of the (Al)GaInP layer, i.e., the lattice mismatch (.DELTA.a/a) (here, a is lattice constant of GaAs, and .DELTA.a is difference between the lattice constant of GaAs and that of the AlGaInP layer) approach to zero as shown in FIG. 8 and, further, it is thought necessary to adjust the lattice mismatch within .+-.0.001. Therefore, a lot of proposals are offered to make the lattice constants of the GaAs substrate and the (Al)GaInP layer coincide with each other as much as possible to lower the lattice mismatch. For example, in Japanese Published Patent Application No. 1-202880, a laser diode, in which the lattice mismatch of the n type cladding layer is previously shifted to the plus side (.DELTA.a/a: 0.0005 to 0.001) thereby to reduce the lattice mismatch in the p type cladding layer, is disclosed. On the other hand, Japanese Published Patent Application No. 2-156522 discloses that an AlGaInP layer having good crystal quality is obtained by varying the growth temperature or the constitution ratio (V/III ratio) of the elements supplied at the start of the production of GaInP buffer layer rather than at the conclusion of the production thereof while producing an AlGaInP/GaAs system multi-layer semiconductor laser on a GaAs substrate. Further, Japanese Published Patent Application No. 2-254715 discloses that the lattice constant of the AlGaInP system layers are made coincide with the lattice constant of a GaAs buffer layer by differentiating the growth temperatures of the layers.
The prior art laser diode shown in FIG. 7 is produced through three crystal growth processes comprising a first growth before producing a mesa, a second growth after producing the mesa, and a third growth for producing a contact layer. FIGS. 9(a) and 9(b) show profiles of Zn as p type dopant in respective layers measured by secondary ion mass spectroscopy (hereinafter referred to a SIMS) in the prior art laser diode shown in FIG. 7. More specifically, FIG. 9(a) shows a profile of Zn from SIMS measurements at the conclusion of the first growth and FIG. 9(b) shows a profile of Zn from SIMS measurements at the conclusion of the third growth. In these figures, the profile of Zn falls steeply at the interface between the undoped GaInP active layer 14 and the p type AlGaInP waveguide layer 15 after the first growth while Zn accumulates at the interface between the undoped GaInP active layer 14 and the p type AlGaInP waveguide layer 15 after the third growth. In this way, the prior art laser diode is produced through a plurality of crystal growth processes and even when diffusion of a dopant impurity into the active layer is not seen after the first growth, the impurity sometimes diffuses into the active layer by such influences as substrate heating during crystal growth in the second and third growth steps. Especially, Zn as p type dopant is abnormally accumulated at the interface between the p type layer and the undoped GaInP active layer and significantly adversely affects lasers characteristics.